One very important source of helium is as a component in the gas stream from a natural gas reservoir. Because of the wide difference in the volatility of natural gas, which is essentially methane, and the volatility of helium, it is relatively easy to separate helium from natural gas.
Often, however, the gas stream from a natural gas reservoir also contains a significant amount of nitrogen. The nitrogen may be naturally occurring and/or may have been injected into the reservoir as part of an enhanced gas recovery or enhanced oil recovery operation. In this situation the gas stream from the reservoir, after certain precleaning operations to remove acid gases, water, and/or higher hydrocarbons, is passed to a nitrogen reaction unit or NRU wherein the gas stream is separated into methane-richer liquid, helium-richer vapor and nitrogen-richer fluid. The nitrogen fraction may comprise from 10 to 70 percent of the feed to the NRU.
The helium-richer vapor is generally upgraded to a higher helium concentration by one or more low temperature partial liquefactions and by ambient temperature purification by pressure swing adsorption and the pure helium gas is then passed to a helium liquifier for liquefaction. Because of the extremely low boiling point of helium, the helium liquefaction requires the expenditure of a larqe amount of power.
It is therefore an object of this invention to produce helium qas from a nitrogen-containing natural gas stream which is suitable as a feed for a helium liquifier and which will enable the helium liquifier to operate with reduced power requirements.